This invention relates to a voltage-dependent resistor suitable as surge-absorbing element.
Voltage-dependent resistors, namely varistors, have been widely used for voltage stabilizing and surge absorbing. The electrical characteristics of such a non-linear resistor are expressed by the relation: EQU I = (V/C).sup..alpha.,
where V is the voltage across the resistor, I is the current flowing through the resistor, C is a constant. It is ordinary desirable that the value of varistor non-linear exponent (hereinafter referred to as exponent ) .alpha. be as large as possible since this exponent determines the extent to which the resistors depart from ohmic characteristic.
For such voltage dependent resistor, varistors of silicon carbide type, which utilize voltage sensitivity of contact resistance of silicon carbide grains, have been widely used. This device has the merit of cheap in cost, but their varistor non-linear exponent .alpha. is so low as 3 to 7, and accordingly, effects in voltage stabilization or surge absorbing are not satisfactory.
For varistors having larger values of exponent .alpha., the varistors containing mainly zinc oxide have been recently developed and put into practical use. The zinc oxide type varistor is made by mixing ZnO small amounts of Bi.sub.2 O.sub.3, PbO and BaO together, moulding in a cast and fired in the air at temperature of 1000.degree. C to 1500.degree. C. The non-linear voltage-current characteristic is produced by the interfaces mainly consisting of additives surrounding sinterred grains mainly consisting of zinc oxide, and the exponent .alpha. of above 50 is obtainable. In the conventional varistors wherein their electrodes are formed by sputtering aluminum and copper on both principal faces of the varistor bulk, though the exponent .alpha. are great, the varistor voltages across their terminals are easily deteriolated by D.C. loading.
In order to improve the D.C. loading life, for the method of forming electrodes on the varistor bulk, a method of applying glass-containing slurry on the varistor bulk and baking and thereafter sputtering Al and Cu, or another method of applying a silver paint containing glass-frit and silver powder on the varistor bulk and baking it has been used. Varistors produced by the abovementioned way shows satisfactory characteristic for voltage stabilizer and are widely used. (e.g. U.S. Pat. 3,962,144).
Recently, the abovementioned zinc oxide type varistors are found to have satisfactory characteristic also for surge absorbing and they become widely used for surge absorbing use.
The abovementioned conventional electrodes for the zinc oxide varistors have been developed and suitable for the use as the voltage stabilizer of high exponent values and durabilities in static D.C. use, but they have contained over 10% by weight of B.sub.2 O.sub.3 and hence have not been suitable for the surge absorbing use. Such high amount of B.sub.2 O.sub.3 is considered as the cause of the hard deterioration of the varistor voltage after impulse current test.
For a surge absorbing element, a characteristic that the deterioration rate of varistor voltage (across its electrodes) should be small even after applications of impulse current or surge current is required.
Also for the index of indicating high capability of the surge absorbing, the ratio V.sub.10A /V.sub.1mA of the voltage V.sub.10A corresponding to the varistor current of 10A to the voltage V.sub.1mA corresponding to the varistor current of 1mA is observed, by using surge currents of special waveform wherein duration of wave-front is 8 microseconds and duration of wave-front tail-length is 20 microseconds (such wave is hereinafter referred to as 8 .times. 20 .mu.s). The ratio V.sub.10A /V.sub.1mA is called "voltage clamping ratio", and the closer to 1 the clamping ratio is, to the extent of the larger current the non-linear characteristic of the varistor exists, hence the better surge absorbing ability the varistor has. In the conventional varistor for the voltage regulation purpose, the clamping ratio has been about 3.
Furthermore, the varistor of the surge absorbing purpose must be stable also for stationary D.C. or A.C. operation.